ng serve --configuration=en throws error

### Versions

Output from: `ng --version`.

Angular CLI: 6.0.3
Node: 8.11.1
OS: darwin x64 (Sierra)
Angular: 6.0.2
... animations, cdk, common, compiler, compiler-cli, core, forms
... http, language-service, material, platform-browser
... platform-browser-dynamic, router

Package                           Version
-----------------------------------------------------------
@angular-devkit/architect         0.6.3
@angular-devkit/build-angular     0.6.3
@angular-devkit/build-optimizer   0.6.3
@angular-devkit/core              0.6.3
@angular-devkit/schematics        0.6.3
@angular/cli                      6.0.3
@ngtools/webpack                  6.0.3
@schematics/angular               0.6.3
@schematics/update                0.6.3
rxjs                              6.1.0
typescript                        2.7.2
webpack                           4.8.3
    
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**### Repro steps**
Step 1:
I did add this to my angular.json file for i18n translation - it's based on the doc [here](https://angular.io/guide/i18n#merge-the-completed-translation-file-into-the-app): 

`...
"serve": {
          "builder": "@angular-devkit/build-angular:dev-server",
          "options": {
            "browserTarget": "demo:build"
          },
          "configurations": {
            "production": {
              "browserTarget": "demo:build:production"
            },
            "en": {
                "aot": true,
                "outputPath": "dist/en/",
                "i18nFile": "src/assets/i18n/messages.en.xlf",
                "i18nFormat": "xlf",
                "i18nLocale": "en"
            }
          }
    },
.......`

Step 2:
Run the command: `ng serve --configuration=en --port 4900`

Step 3: Run/try other command: `for lang in en de; do ng build --output-path=dist/$lang --aot --prod --i18n-file=src/assets/i18n/messages.$lang.xlf --i18n-format=xlf --locale=$lang; done`


**### Observed behavior**
When I run the command: `ng serve --configuration=en --port 4900`, I get the following error:

> Schema validation failed with the following errors:
> Data path "" should NOT have additional properties(outputPath).

When I run the following: `for lang in en de; do ng build --output-path=dist/$lang --aot --prod --i18n-file=src/assets/i18n/messages.$lang.xlf --i18n-format=xlf --locale=$lang; done`, I get the following error:

> Unknown option: '--locale'

**### Desired behavior**

What would like to see implemented?
i18n Internazionalization  for different languages - in this case EN and DE

What is the usecase?
Serving the App in specific language based on the serve command mentioned above.

What did you expect to see?
Serving the App in specific language based on the serve command mentioned above.

[ganchuhang]

Any proper docs that guide us on ng serve with configurations in angular.json?

[chimung]

in my configuration of “qa” in serve. I reference by “name-of-your-project:build:qa”. It will get configuration of “qa” in build